8 Elementary Zoology. 



undoubtedly are, they really prove nothing, since protoplasm is 

 admittedly not an oily substance, though it may often contain 

 oil drops formed by its own activity. 



Having considered the protoplasm which builds up the bulk 

 of the amoeba's body, we may turn to the nucleus. The nucleus 

 is a spherical to elliptical structure, encircled by a definite wall, 

 and showing a granular appearance. One or more rounded 

 bodies in its interior are termed nucleoli. At one time the 

 nucleus was regarded simply as a denser bit of protoplasm in 

 the interior of the protoplasm, denser perhaps by reason of its 

 central position and the consequent pressure of the surround- 

 ing substance. It is now known to be a perfectly distinct 

 structure. This opinion is based upon its minute structure, 

 which has been of late years more elaborately investigated, and 

 by the physiological importance which it has been proved to 

 possess. This is evidently incompatible with a mere central 

 thickening of the protoplasm. The nucleus very commonly 

 exhibits a reticular arrangement of its contents, the denser 

 network consisting largely of a substance which has been 

 termed chromatin. But the constitution of the nucleus is 

 more fully entered into below (p. 127) in connection with its 

 multiplication. As to the importance of the nucleus, it seems 

 probable, in the first place, as already stated, that a nucleus 

 is always present in living bodies. There are apparently a 

 few exceptions, such as those minute organisms so often con- 

 nected with disease, and generally known as bacteria. Figures 

 showing a structure like nuclei in those organisms have been 

 published, but not to the satisfaction of everybody. The view 

 has been advanced that the whole body of the minute plant is 

 a free nucleus with but a slender rim of protoplasm. This may 

 be the case, but the matter cannot be fully gone into here ; it is 

 sufficient for the present purpose to insist upon the certainly 

 almost (and probably quite) universal presence of a nucleus. 

 The importance of the nucleus in the protoplasm is shown by 

 the striking part that it takes in the division of the amoeba; 

 it initiates this division. Experiments have been made with 

 large amoebse, and with allied organisms, which tend to show 

 that when the creature is torn up by fine needles into small 



